Attic subpanel

I am finishing out about 900 sq ft of my second floor that is currently being used as attic. The framing is complete. I will have a 18.5'x30' theater with a wet bar and a bedroom in this space. I currently only have a 20 amp GFCI outlet and some keyless lights that are on a shared circuit with my second floor lights. I would like to run a 90 amp sub panel into an unfinished storage space adjacent to the theater. My plan is to run the wire up from the subpanel to the ceiling joist, down that to the edge of the house, out of the soffit and straight down the side of the house, then punch into the crawl space and come out right under the existing 200 amp service panel. The section from the soffit to the crawl space would be run inside of 1 1/2 PVC conduit. Do I need a J box at the soffit and at the bottom when it goes into the crawl? My big question is what size and type of wire do I need for this? I might only wire up 80 amps but I would like to leave myself some headroom so just pretend it'll be 90 amps now.

I have not done a load calc. That statement was just based off of 3 electricians that cane to bid the job telling me they though the max available would be 90. I don't know exactly how many amps I'll need.

I have not done a load calc. That statement was just based off of 3 electricians that cane to bid the job telling me they though the max available would be 90. I don't know exactly how many amps I'll need.

They need to do a load calculation, they cant go off the top of their head. You could put in a 200 amp sub panel in a 60 amp main if u wanted to as long as the loads dont exceed 60

As eastcoastsparky said, you gotta do a load calc to know absolutely, but you should be fine with a 100A sub on a 200A service.

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Thats true if a customer had a 200 amp service i wouldnt even think of doing a load calc but if its a 100 amp service i would highly recommend it. Now the ultimate question is what size service does the OP have

From what you are planning to use it's unlikely you require more than a 70 or perhaps 80 amp sub for that space. 6 or 8 circuits will be plenty for you and give you room for some expansion. Use conduit for the complete run and pull ells if you need to keep it continuous. #6 copper for 60 amp and #4 for 80 will work. Are you having the electricians do this or are you going to do it yourself? Some grounding issues will have to be done but your electrician will know how to bond the sub and isolate the neutral buss.